In a general sense, this invention relates to braze compositions. More specifically, it relates to braze materials suitable for repairing nickel-based superalloy articles, or for joining different sections of superalloy components.
Nickel- and cobalt-based superalloys are very important in a number of industrial applications. These applications often involve extreme operating conditions, wherein the superalloys may be exposed to high temperatures, e.g., above about 750° C. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating. Gas turbine engines are a prime example of components subjected to such an environment.
It is frequently necessary to join various superalloy components together, and brazing techniques are often preferred to accomplish this objective. As an example in the case of power turbines, brazing may be used for a variety of critical components, such as turbine seals, first-stage turbine nozzle guide vanes, and turbine blades. These types of parts are often subjected to high temperatures and high-oxidation conditions in service.
A wide variety of braze compositions are commercially available, and can be adjusted to meet the requirements for the particular components being joined. As one example, U.S. Pat. No. 6,165,290 (Rabinkin) describes brazing materials based on cobalt-chromium-palladium-type alloys. The materials are said to be useful for brazing superalloy components which operate in a high temperature service environment. As another example, U.S. Pat. No. 4,414,178 (Smith, Jr. et al) describes nickel-palladium-chromium-boron brazing alloys. Those materials are useful for brazing in the 1800-2000° F. (982-1093° C.) temperature range, and exhibit good flow and “wettability” characteristics.
In addition to their use in the formation of joints, the braze compositions are often employed as repair materials. For example, they can be used to fill cracks, cavities, and other indentations within the surface of a superalloy component. U.S. Pat. No. 6,530,971 (Cohen et al) describes a nickel-based repair composition in the form of two powders. One of the powders includes, primarily, chromium, cobalt, titanium, aluminum, tungsten, and molybdenum, in addition to nickel. The other powder contains some of these elements, along with a significant amount of boron, which allows it to melt at a much lower temperature than the first powder. Combination of the two powders results in a braze slurry which effectively melts and fills a crack, but which does not interfere with surrounding features, such as turbine cooling holes.
Another repair process is described in U.S. Pat. No. 6,520,401 (Miglietti). The method involves filling a gap or crack in a metal component, using a liquid-phase diffusion bonding technique. The gap is first filled with an alloy powder having a composition similar to that of the component, and substantially free of melting point depressants. A braze which contains a melting point depressant is then applied over the powder in the gap. A first heating treatment is then employed to bring the temperature above the liquidus of the braze, but below the melting point of the powder. This allows the braze to infiltrate spaces within the powder. In a second heating stage, a temperature below the liquidus of the filler material is maintained, while diffusion of the melting point depressant occurs.
It is readily apparent that many different types of braze compositions are available, and are often used in repairing or joining superalloy components. While each of the commercial compositions may be very suitable for a number of applications, most of them still exhibit some deficiencies when they are used in certain situations. In the past, the deficiencies have often been of minimal importance, in view of the overall advantages of the braze materials. However, recent trends in various industrial segments have served to highlight some of those deficiencies.
In a very general sense, there are two primary requirements for the braze materials (whether used in a joining process or a repair process). First, they should be capable of being applied effectively to the component(s), e.g., with sufficient flow and wettability characteristics. Second, they must be capable of eventually solidifying into a joint or fill-material with adequate physical properties, such as strength, ductility, and oxidation resistance.
As technical requirements and other industrial needs increase, it is becoming more difficult to satisfy these general requirements for brazes. For example, many brazing operations for gas turbine components continue to require braze materials with demanding flow characteristics. The materials must also melt at temperatures low enough to protect the base material or workpiece from becoming overheated or otherwise damaged. It is therefore often necessary to incorporate significant amounts of metalloid elements such as boron and silicon into the braze compositions.
However, significant levels of boron and silicon can be detrimental to the final braze product. For example, these elements tend to form brittle, intermetallic phases in the braze micro structure. The Miglietti patent mentioned above alludes to this problem in the case of boron, e.g., describing the loss of ductility due to the presence of boride phases like Ni3B.
Moreover, the Rabinkin patent describes other adverse effects caused by the use of boron and silicon metalloids. High-temperature components used in turbine parts, for example, often obtain their oxidation resistance via the formation of a dense alumina or alumina/titania protecting film on the surface of the component. If the components are subjected to brazing operations which contain the metalloids, the protecting film in the brazed region can be partially or completely damaged. As a result, the brazed interface can act as a conduit for oxygen penetration, leading to oxidation-attack of the entire part.
Continuing developments in industry have made the search for improved braze compositions more problematic. The advent of higher-strength and more highly-alloyed superalloy materials has created the need for brazes which are more closely matched with the superalloy. Specifically, the braze material often has to have a microstructure that is closely matched with the microstructure of the base alloy, while still exhibiting the high strength needed for many industrial applications.
Moreover, in the case of equipment like gas turbine engines, standard operating temperatures continue to be increased, to achieve improved fuel efficiency. This trend increases the propensity for corrosion and oxidative attack of the turbine components, i.e., the superalloy materials from which the engines are made. While steps are taken to improve the base alloys or otherwise protect them from this damage, steps also have to be taken to ensure the integrity of any brazed regions within the turbine engines.
It should thus be evident that new braze compositions for use with nickel-based superalloys would be very welcome in the art. The compositions should have a melting point low enough for many current brazing operations (e.g., for turbine engines). They should include only restricted amounts of elements such as boron and silicon, which can produce secondary phases in the final braze, or which can otherwise decrease braze integrity.
Moreover, the braze compositions should have flow and wettability characteristics which facilitate joint-forming or cavity-filling processes. The compositions should also be generally compatible with the component being brazed, e.g., in terms of microstructure. Furthermore, after solidifying, the braze compositions should exhibit the necessary characteristics for a given end use application, e.g., a desirable level of strength, ductility, and oxidation resistance.